Sorry, but I really don't think that's it.
Point 1 - I've been having the problem since before this virus.
Point 2 - the virus slows down email delivery of legitimate emails as infected emails are filtered by email hosts.
Point 3 - it doesn't explain the sporatic nature of why email notifications of new posts in some threads are received and some aren't. i.e. some threads to which I've subscribed I haven't gotten any notifications, and some to which I've subscribed I've gotten them all.
Point 4 - I'm not having the problem with a vBulletin board I'm on.
Point 5 - I am having the problem with another phpBB board I'm on.
When I first started having this problem, I browsed through threads about email notification problems in the phpBB forum. There seemed to be quite a few on this situation. My previous comment about a phpBB gremlin came from one of those threads.
Naw....it's not the Sober virus.